Method for producing crystal and/or crystal materials containing fluorine

ABSTRACT

A method for producing crystals and/or crystal materials is described whereby the undesirable impurities are removed from the crystal and/or the material with the aid of a purification agent or getter. Elemental fluorine and/or a reactive fluorine-containing substance is used as the purification agent or getter. Preferred getters are XeF 2  and/or carbon fluoride. Crystals obtained in this manner are suitable for use as optical components.

[0001] The invention relates to a method for producing crystals and/or crystal materials that are free of undesirable impurities, and to the use thereof.

[0002] The demand for high-purity, flawless crystals is constantly increasing. For example, such crystals are needed for modern photolithographic techniques that use light sources of very short wavelength. The preparation of such optical systems for use in photolithographic devices with short-wave lasers, for example lasers having a wavelength A of about 157 nm, is possible only if materials that are optically flawless for these wavelengths are available in sufficient dimensions. For certain applications, crystalline CaF₂, for example, proved to be an appropriate material. The yield of single-crystal material of high quality such as is required for optical components in the 157 nm range, however, is usually not sufficient or much too low. Demand is growing not only for crystalline CaF₂ but also for other fluoride materials for optical systems and optical polarization components as well as for appropriately doped optically active media.

[0003] CaF₂ has so far proved to be suitable for the production of single crystals used as starting material for the aforedescribed optical components in DUV [=deep ultraviolet−Translator] photolithography, for example steppers or excimer lasers. Such crystals are usually employed as lenses or prisms and are used, in particular, in the production of electronic devices to copy fine structures optically onto integrated circuits and on computer chips and/or wafers coated with photosensitive compositions. To achieve the image definition or sharpness required for such methods, these optical components must have high homogeneity, namely their refractive index must be highly constant throughout the entire crystal. This means that the refractive index throughout the crystal volume may show a difference An of at the most 5×10⁻⁶ and preferably at the most 2×10⁻⁶.

[0004] It has been established that in the growing of crystals, for example of CaF2 the material from which the crystal is grown requires purification, because even the slightest amount of impurities in the ppm range, for example of oxygen and water, which for technical reasons are necessarily introduced from raw materials and the crystal-growing apparatus, can cause undesirable chemical side reactions in the melt of the starting material for the crystal. Such side reactions lead to the formation of crystal defects by incorporation of foreign atoms or ions into the crystal lattice thus causing a deterioration of the optical properties of the finished crystal. Substantial deterioration is observed particularly in terms of the transmission performance, the scattering and the radiation resistance of the crystals, especially at wavelengths below 200 nm.

[0005] A procedure used until now for removing the aforesaid impurities or for purifying the starting material is in the case of fluoride crystals, for example CaF₂, the addition of PbF₂. In fact, it has been shown that the addition of PbF₂ brings about chemical reactions which—depending on the process temperature and, hence, on the vapor pressure of PbF₂—lead to partial removal of oxides, hydroxides or water. By this process, and mainly in a temperature range between 600 and 900° C., substances are formed which are already volatile several degrees centigrade below the melting point of CaF₂, for example HF and PbO. In this manner, the impurities vaporize or sublime into the atmosphere surrounding the crystal wherefrom they can be removed in simple fashion by suction. The addition of PbF₂ to a fluoride crystal, for example CaF₂, thus contributes substantially to the formation of a crystal melt low in oxides or oxygen and to the growing of CaF₂ single crystals of high quality. Such a procedure is described, for example, in K. Th. Wilke, “Kristallzuchtung” [The Growing of Crystals], Deutscher Verlag der Wissenschaft [publisher], Berlin, 1988, p. 630 ff.

[0006] For the highest requirements, especially in the 157-nm range, the addition of PbF₂ as a getter of impurities, is not advantageous. This is because the danger exists that in industrial production PbF₂ or its reaction products, for example PbO, would vaporize only incompletely leaving Pb²⁺ ions in the finished crystal. This would lead to optical absorption bands which, for example in CaF₂ crystals, besides absorbing at 220 nm would also strongly reduce transmission in the 150 nm range.

[0007] Hence, to solve this problem it has already been proposed to use ZnF₂ in place of PbF₂. This method, however, is also not particularly well suited, because, here, too, complete vaporization of the getter material cannot be guaranteed and especially because, for example, ZnF₂ and the ZnO derived therefrom vaporize at higher temperatures than PbF₂ and PbO. Moreover, Zn residues also cause absorptions in the short-wave range from 150 to 170 nm.

[0008] The object of the invention therefore is to provide a method whereby it is possible to remove completely undesirable impurities from the starting material and/or from the surfaces of the components of the apparatus, and, in particular, to bring about complete drying and to prevent or suppress hydrolysis reactions. As a result, undesirable side reactions, for example at the surfaces of the starting material for the crystal, are at the same time reduced to a minimum.

[0009] According to the invention, we have now found that this objective can be reached during the production of such crystals and/or crystal materials by removing the undesirable impurities by use of elemental fluorine or F₂. According to the invention, this is preferably achieved by maintaining an F₂-containing atmosphere in the drying, melting, growing and/or annealing apparatus. In fact, surprisingly, it has been found according to the invention that even the lowest partial pressures of F₂ are sufficient for achieving the objective of the invention. Advantageously, the atmosphere contains additional gases which optionally act as carrier gases for the gaseous fluorine. In particular, the atmosphere contains no constituents capable of reacting with fluorine under process conditions. Particularly preferred atmosphere and carrier gases are the noble gases, with helium, argon and/or xenon being particularly preferred. Vacuum can also be used as the atmosphere provided that it does not cause intolerable vaporization of the actual crystal material.

[0010] In principle, it is also possible to introduce elemental, namely free fluorine directly into the production unit, it being preferable, however, to use fluorine together with a carrier gas. Because of the high cost of safety measures for and the high reactivity of fluorine, however, it has been found advantageous to add directly to the starting material fluorine-generating and/or reactive fluorine-containing substances or to introduce them into the production unit at a separate point. The fluorine or the substances generating or containing it are preferably introduced at the bottom of the production unit so that the fluorine moves upward and flows through or around the material to be purified. Particularly preferred, however, are solid or liquid fluorine-liberating or containing substances. Most preferred is the use of XeF₂ which is solid at room temperature but on heating decomposes into Xe and F₂ gas. Particularly preferred among the substances containing reactive, particularly covalently bound fluorine are the fluorocarbons, particularly C₁-C₈ and especially C₁-C₃ fluorocarbons.

[0011] Suitable gases for the atmosphere prevailing under the process conditions of the invention are, besides the aforesaid noble gas—gaseous fluorine mixtures, also gases consisting of gaseous mixtures of or with fluorocarbons, for example CF₄ and/or C₂F₆, C₃F₈ etc. Surprisingly, the otherwise so inert and, like the noble gases, chemically stable fluorocarbons react directly with oxide or hydroxide impurities. Thus, they act as fluorine-liberating substances and, hence, for purposes of the invention are to be understood as such. The reactions are preferably carried out at elevated temperatures. The reaction products thus formed from the impurities, for example CO₂, carbonyl fluoride and HF, are highly volatile and, hence, can readily be removed by applying suction.

[0012] In many cases, it has been found advantageous to use other, particularly HF-liberating substances, especially solid or liquid substances which on heating in the temperature range of up to 350° C. undergo complete decomposition into volatile components with liberation of HF, for example NH₄F.HF and/or triethylamine trihydrofluoride (CAS 73602-61-6). This approach can also be used for the preparation or recycling of alkyl fluorides.

[0013] The method of the invention causes the volatilization of impurities even at low temperatures, which, for example, with fluorine/noble gas mixtures already occurs in the low temperature phase from room temperature (20° C.) to about 350° C., or with fluorocarbons, optionally together with a noble gas, from 600 to 1500° C., and particularly above about 800° C. It is, of course, possible to work with fluorine/noble gas mixtures also above, or with fluorocarbons below, the indicated range even though this is not preferred. In principle, according to the invention, it is also possible to introduce pure gaseous F₂ as the active component already in the drying phase of the unit and of the starting materials, particularly before the actual growing process.

[0014] In fact, we have found, surprisingly, that when free fluorine is used, the undesirable components are nearly completely transferred into the gas phase and are thus removed from the starting materials used, namely from the crystal and/or the crystal raw material. Water, hydroxides and oxygen, in particular, are completely removed.

[0015] Moreover, we have found that the use of F₂ according to the invention gives definitely better results than when HF is used according to the prior art, for example in the form of NH₄F.HF.

[0016] According to the invention, we have also found that, for example in the case of a fluorine/noble gas mixture, an amount from 0.1 to 20 wt. % and preferably from 2 to 5 wt. % of fluorine in the gas mixture brings about the desired purification at a temperature between 25 to 100° C. without causing damage to the process unit. In this case, the gas mixture is preferably passed from the bottom of the unit through the powdered starting material or starting material comminuted into fragments to bring about a reaction with the residual moisture. The afore-indicated mixing proportions can also be used for mixtures of noble gas and reactive fluorine-liberating substances,

[0017] Because the handling of elemental, i.e., free gaseous fluorine requires an increased expense for safety measures, the method of the invention is carried out in a temperature range of up to a maximum of 350° C. and preferably up to a maximum of 100° C.

[0018] Xenon difluoride (XeF₂) which is preferred according to the invention also affords F₂ even at low temperatures so that in this case, too, undesirable impurities are removed and released into the surrounding atmosphere even at room temperature. Moreover, for example, the undesirable oxidized starting material is converted into the corresponding fluoride which thus in the form of pure material once again becomes available for crystal growing. XeF₂ itself vaporizes almost completely at temperatures above 200° C. and with rising temperature decomposes into Xe and F₂. By slow heating under vacuum before the beginning of crystal growing, an optimum post-purification of the starting material is thus possible. This gives higher yields of materials or single crystals which better satisfy the objectives of optical use in terms of absorption, scattering and/or radiation resistance than do the products obtained by prior-art methods. In an advantageous embodiment of the invention, the spent xenon gas is collected and made to react with fluorine to form XeF₂, which is then available for reuse. Xenon itself can be collected, for example by means of a cold trap, and made to react with gaseous fluorine in the known manner, for example at 300-400° C. in a coiled tube, to form XeF₂.

[0019] Another advantage of the method of the invention lies in that the predrying takes place at relatively low temperatures so that even compounds with a low melting point can be post-purified. In this manner, it is also possible, according to the invention, to purify or dry laser crystals from the family of coloquiriite materials, for example LiCaAlF₆ and LiSrAlF₆, in a fluorine atmosphere thus enabling them to be grown with high perfection. Particularly in view of the long processing times of a few weeks required for crystal growing and annealing, it is not possible, or it is possible only with difficulty, to keep PbF₂, which until now has commonly been used as purification getter, in the process for its entire duration, because in the range from 800 to 1000° C. PbF₂, until now commonly used for purification, has a high vapor pressure. Surprisingly, however, this becomes possible by adding fluorine-containing or fluorine-liberating gas mixtures, such as fluorocarbons and noble gases, which later at higher temperatures can be introduced into the evacuated processing unit during the entire processing time. In this manner, it is possible, even if processing times are long, to obtain materials which show no deterioration in absorption, something which until now, by using PbF₂, was not possible.

[0020] The substances containing or liberating elemental or reactive fluorine can be introduced into the melting crucible as reactive gas together with the as yet unmolten starting material through gas inlet tubes and the recipient and/or, after the starting materials have melted, through gas lances directly into the melt.

[0021] The method of the invention can be applied during the entire course of the process for producing optical crystal materials, namely during the drying phase of the starting material as well as during the scavenging phase by means of known getters, such as PbF₂ and ZnF₂, as well as during the melting phase, growing phase and subsequent cooling phase and optionally during the following annealing phases.

[0022] The crystals or crystal raw materials obtained by the method of the invention are particularly well suited for use in the production of lenses, prisms, fiber-optic rods, optical windows and optical components for DUV photolithography, steppers, lasers, particularly excimer lasers, wafers, computer chips as well as integrated circuits and electronic devices containing such circuits and chips.

[0023] The invention will now be explained in greater detail by way of the following example.

[0024] About 2 kg of powdered CaF₂ starting material was mixed with 1 kg of XeF₂ and the mixture was distributed over the bottom of a crucible. The remaining volume of the crucible was then filled with a total of 48 kg of a mixture of CaF₂ and PbF₂, and the crucible was placed in an inductively heated furnace. Preferably, the crucible itself is heatable.

[0025] By placing the XeF₂ on the bottom of the crucible, optimum reaction of the liberated fluorine with the fluoride to be processed is achieved. The entire crucible system is located in a gas-tight container which optionally can be pumped down or evacuated through pump connections.

[0026] On heating, XeF₂ decomposes with formation of Xe and F₂. Subsequently, a single crystal can be produced by known methods from the molten fluoride being processed.

[0027] In the conventional crucible system for growing CaF₂ single crystals, XeF₂ must be introduced in a manner such that its decomposition products—and above all the reactive F₂—can flow through the, particularly powdered, fusible material. By an accurate quantitative limitation of XeF₂, which can be weighed in the solid state, it is thus possible to achieve good dosing of free fluorine and thus a definite reduction in its corrosive action on the materials in the gas-tight container.

[0028] The F₂/noble gas mixture together with the starting material to be dried is introduced either into an appropriately pre-evacuated reaction vessel made of stainless steel which after being closed is transferred to the processing unit, or the mixture together with the starting material is placed directly in the pre-evacuated production unit, preferably directly on the bottom of the melting crucible. By repeated evacuation/addition, gas flow through the powdered starting material is also ensured under certain circumstances. In principle, it is also possible to introduce XeF₂ into a container which can be destroyed by heat and which is made of material that does not interfere with the process. Such a container is, for example, a Teflon bag.

[0029] The amount of PbF₂, when present, was 3 wt. % of the amount of CaF₂. XeF₂ was added as a solid in an amount of 2 wt. %. The reactive getter gases F₂ and CF₄ were introduced into the heating container at 500 mbar at a rate of 80 L/h (based on normal conditions under pressure and at room temperature).

[0030] After the initial high vacuum of <10⁻⁶ mbar, the optimum pressure range for the unit increases to 10⁻⁵ to 100 mbar as a result of the addition of reactive gases from the outside, particularly at temperatures above 800° C. In case of a melting batch of 50 kg of crystal raw material and working in the low pressure range of 10⁻⁵ to 10⁻⁴ mbar, 1 to 10, preferably 3 to 7 and particularly 4 to 5, L/h of reactive gas was introduced. When working in the pressure range of 100 to 800 mbar and particularly 400 to 600 mbar, 150 L/h and preferably up to 100 L/h was introduced into the crystal-growing unit.

[0031] From such 50-kg batches crystals were produced as described hereinabove under a comparable time/temperature regime, and from the crystals were prepared 100-mm-thick material samples which has a diameter of 25 mm and was taken from a comparable volume element of the crystals. The samples were then annealed for ten days at 1200° C. in a CF₄ gas atmosphere, after which their end surfaces were ground and polished for the purpose of subjecting them to optical absorption tests.

[0032] The test results for different samples are indicated by different scattering losses caused by embedded, microscopically small CaO deposits: Test Conducted Under Absorption Coefficient Test No. the Following Conditions k (L/cm) at 193 nm 1 Standard CaF₂ with PbF₂ 0.003 2 As in 1., plus XeF₂ 0.0007 3 As in 2., without PbF₂ 0.0005 4 As in 1., plus F₂ 0.00004 5 As in 4., without PbF₂ 0.00007 6 As in 1., plus CF₄ 0.00009 7 As in 6., without PbF₂ 0.0001 

1. Method for producing crystals and/or crystal materials whereby undesirable impurities are removed from the crystal and/or the material with the aid of a purification agent or getter, characterized in that elemental fluorine and/or a covalently bound reactive fluorine-containing substance is used as purification agent or getter.
 2. Method according to claim 1, characterized in that it uses a fluorine-containing atmosphere.
 3. Method according to one of the preceding claims, characterized in that the fluorine is generated from an F₂-liberating agent in situ in the course of the process.
 4. Method according to one of the preceding claims, characterized in that the fluorine-liberating agent is XeF₂ and/or a carbon fluoride.
 5. Method according to claim 4, characterized in that the spent Xe is separated, again converted to XeF₂ and reused.
 6. Method according to one of the preceding claims, characterized in that the crystal material is BaF₂, MgF₂, LiF or KMgF₃, CaF₂ or an appropriate, doped crystal such as, for example, LiCaAlF₆ and/or LiSrAlF₆.
 7. Method according to one of the preceding claims, characterized in that the fluorine is introduced or liberated at the bottom of a vessel containing the crystal and/or crystal material.
 8. Use of crystals and/or crystal materials obtained by one of the methods according to claims 1 to 7 for producing lenses, prisms, fiber-optic rods, optical windows and optical components for DUV photolithography, steppers, lasers, particularly lasers, wafers, computer chips as well as integrated circuits and electronic devices containing such circuits and chips. 